During the past few years we have developed high resolution NMR as a powerful new tool for investigating the structure of polynucleotides in solution. In the present proposal we discuss ways in which these developments will be used to investigate in detail a number of questions regarding the conformation of tRNA molecules in solution. This will include studies of the effect which a variety of environmental factors (temperature, pH, ionic strength, divalent metal ions, drugs, chemical modification) have on the conformation of tRNA, and the relation between the secondary and tertiary structures of different tRNA from the same relation between the structure of yeast tRNA Phe in the crystal and in solution. Some biochemical studies will also be carried out in order to correlate structure with function. Another important part of the project will be to investigate the structure of 5S RNA from different organisms and the secondary structure of messenger RNA. The significance of this work is several-fold. First NMR is being further developed as a tool for investigating the secondary and tertiary structures of polynucleotides in solution. Secondly, we have chosen for study a collection of biologically important molecules (tRNA, 5S RNA and messenger RNA) where secondary and tertiary structure are believed to be important to function.